1. Materials Science
Lesson 6

2. RECALL FROM PREVIOUS STUDIES
NAMING MOLECULES:
Binary compounds: These are compounds that contain only 2 elements. When naming binary compounds we use prefixes to indicate the amount of each element present
*Mono is only used for the second element in a compound. 1 2 3 4 5 6 7 8 9 10
Mono* di
tri
tetra
penta
hexa
hepta
octa
nona
deca

3. RECALL
Ex. CO P4O CF4

4. RECALL CHEMICAL FORMULA:
Use the prefix to determine the number of each element and then write it down:
Dinitrogen trioxide
Diphosphorus pentoxide

5. Electronegativity:
Electronegativity is the ability of an atom to attract electrons to it when bonded
The electronegativity scale was created by Linus Pauling in 1922 to help develop a better understanding of chemical bonds
We can compare electronegativity values between atoms to determine the type of bond that will form between them

6. Electronegativity difference Most probable bond type
An element with a high electronegativity value is really good at attracting electrons and an element with a low electronegativity is not.
In general, electronegativity increases as you move from left to right across the periodic table and decreases as you move from top to bottom.
Electronegativity difference
Most probable bond type
Nonpolar covalent
Moderately polar covalent
(ish)
Very polar covalent
>1.7
ionic

8. Non Polar Bonds
If both atoms have an equal (or approximately equal) pull on the electrons (same electronegativity), the bond is considered to be non-polar.
Diatomic molecules are non-polar because they are made up of two atoms of the same element with the same electronegativity and therefore both atoms have equal pull on electrons.

9. Non Polar Bonds
Ex. Cl2
Electronegativity difference:

10. Polar Bonds
When two covalently bonded atoms have a significant electronegativity difference (between 0.4 and 1.7) the electrons are shared unequally.
The atom with the higher electronegativity attracts electrons more strongly and therefore gains a slightly negative charge. The atom with the lower electronegativity gains a slightly positive charge.
The slightly positive and slightly negative parts of the molecule are called poles. Because polar covalent molecules contain a positive pole and a negative pole, they are called dipoles.

11. Polar Bonds
Ex. HCl
Electronegativity difference

12. Polar Bonds
Since many molecules have more than one bond we have to consider all bonds in a molecule to determine polarity.
The shape of a molecule determines the polarity of the molecule. If the molecule contains polar bonds and the shape is symmetrical the molecule will be non polar. If there is asymmetry the molecule could be polar.

13. Polar Bonds
Ex. CO vs H2O

14. Polar Bonds
See Polarity Assignment